The present invention relates to various architectures for creating a MEMS microphone biasing network with an adjustable output.
MEMS capacitive microphones operate utilizing conservation of charge. The amount of charge placed on the capacitor is set by a biasing circuit which pre-charges the capacitor. The amount of signal produced for a given capacitance and the change in capacitance in the presence of an acoustic signal is directly proportional to the amount of charge placed on the capacitor.
A high impedance switch network, usually consisting of two anti-parallel diodes with a MOS transistor in parallel with the diodes, is used to apply a fixed charge across two plates of a capacitor. When the microphone is initially turned on, the MOS transistor is switched on allowing a DC voltage to be put on one plate of the capacitor while the other plate is held at a different electrical potential. When the capacitor is fully charged the MOS transistor is switched off and the capacitor is left with a fixed charge across the two plates. When sound pressure waves hit the moveable plate of the capacitor, the capacitance changes. Since the charge is fixed, the voltage increases or decreases proportionally to the amount of capacitance change that is induced by the incident sound pressure.
Process variations in the sensor can affect the amount of electrical signal produced for a given sound pressure level between different microphones. Two of the most significant variations include the stiffness of the movable plate of the capacitor and the gap between the plates of the capacitor. With a fixed bias voltage these variations not only affect the sensitivity of the microphone but also the electrostatic forces on the sensor.